1. Field of the Invention
The present invention relates to a method for treatment of tissue, for example, collagenous tissue, where a deleterious mechanical loading environment contributes to the degradation of the tissue. More specifically, the present invention relates to a method for treatment of degenerated intervertebral discs to improve fatigue resistance, and to non-toxic crosslinking reagents that are effective fatigue inhibitors.
2. Description of the Related Art
Back pain and disability associated with spinal degeneration and instability continue to be one of the costliest and most prevalent health problems in western civilization. Current treatments for spinal instability and low-back pain, including spinal fusion, are generally ineffective in slowing the progression of degeneration. Epidemiological and morphological studies have shown that the capacity of spinal tissue to withstand repetitive loading is one critically important factor in the progression of spinal osteoarthritis (Magora 1972, Kelsey 1975, Frymoyer 1983, Videman 1990).
The organization of collagen and proteoglycans within the intervertebral disc, which is known to those in the art as comprising three major components (the nucleus pulposus, the annulus fibrosus, and a pair of cartilaginous endplates), plays an important role in determining the biomechanical properties of the disc. Biochemical alterations in the structure of the annular matrix affect the disc's durability, that is, its ability to withstand repetitive mechanical loading. Previous studies have shown that nonreducible pyridinoline cross links are predominant in adult cartilage, bone, and intervertebral discs and these collagen crosslinks are thought to be critical for the structural integrity (enzymatic and mechanical) of adult connective tissue (Burgeson and Nimni, 1992, Eyre, 1988). Pentosidine crosslinking has been shown to increase with age in articular cartilage and intervertebral discs (Bank 1998, Pokharna 1998).
A role for naturally occurring crosslinks in stabilizing degenerating discs has been suggested. Duance (1998) noted that while the nonenzymic derived crosslink pentosidine showed an expected age related increase, its level was lower in the more severely degenerated samples. It may be that age related tissue changes—i.e. micro-damage accumulation—combined with inadequate levels of crosslinks made these discs more vulnerable to mechanical degradation. Age related crosslinks (pentosidine) have been shown to increase the strength and stiffness of articular cartilage (Chen 2001) while age related microdamage accumulation would act to decrease strength and stiffness. With regard to viscoelastic properties, Lee (1989) found that aldehyde fixation (crosslinking) reduced stress-relaxation and creep in bovine pericardium, while fatigue loading produced an increase in stress-relaxation and creep in our preliminary testing of intervertebral discs.
Crosslinking reagents are capable of improving the tensile properties of collagen-based biomaterials. Osborne et al (1998) found mechanical strength of acellular collagen gels was most improved using a combination of crosslinking agents. Other researchers have also found that crosslinking treatments can increase the strength of collagenous tissues (Wang 1994, Chachra 1996, Sung 1999, Zeeman 1999). Sung (1999) found that a naturally occurring cross linking agent, genipin, provided greater ultimate tensile strength and toughness when compared with other crosslinking reagents. Genipin also demonstrated significantly less cytotoxicity compared to other more commonly used crosslinking agents. However it also stood out in a negative sense with regard to eliminating tissue anisotropy in bovine pericardium. Several researchers have stated their expectation that crosslinking of collagenous tissue may make the tissue more prone to fatigue failure (Bank 1998, Chen 2001, Kerin 2001). However, it is believed that the opposing view—that crosslinking collagenous tissue may actually benefit fatigue resistance—has not been recorded in the medical literature. It is believed that collagen crosslinks may act as sacrificial bonds to protect collagenous tissues by dissipating energy and improving fatigue resistance.
Fatigue is a weakening of a material due to repetitive applied stress. Fatigue failure is simply a failure where repetitive stresses have weakened a material such that it fails below the original ultimate stress level. In bone, two processes—biological repair and fatigue—are in opposition, and repair generally dominates. In the intervertebral disc, the prevalence of mechanical degradation of the posterior annulus (Osti 1992) suggests that fatigue is the dominant process. Active tissue response (adaptation, repair) does not play a strong role in the case of mature intervertebral disc annular material. As a principally avascular structure, the disc relies on diffusion for nutrition of its limited number of viable cells. Age related changes interfere with diffusion presumably contributing to declining cell viability and biosynthetic function (Buckwalter et al. 1993, Buckwalter 1995). Age related decline in numbers of cells and cell functionality compromises the ability of the cells to repair mechanical damage to the matrix. While regeneration of the matrix in the nucleus following enzymatic degradation has been accomplished, albeit inconsistently (Deutman 1992), regeneration of functional annular material has not yet been realized.
Combined with this limited potential for repair or regeneration, studies have shown that posterior intervertebral disc tissue is vulnerable to degradation and fatigue failure when subjected to non-traumatic, physiologic cyclic loads. Prior work has shown deterioration in elastic-plastic (Hedman 99) and viscoelastic (Hedman 00) material properties in posterior intervertebral disc tissue subjected to moderate physiological cyclic loading. Cyclic load magnitudes of 30% of ultimate tensile strength produced significant deterioration of material properties with as little as 2000 cycles. Green (1993) investigated the ultimate tensile strength and fatigue life of matched pairs of outer annulus specimens. They found that fatigue failure could occur in less than 10,000 cycles when the vertical tensile cyclic peak exceeded 45% of the ultimate tensile stress of the matched pair control. In addition, Panjabi et al (1996) found that single cycle sub-failure strains to anterior cruciate ligaments of the knee alter the elastic characteristics (load-deformation) of the ligament. Osti (1992) found that annular tears and fissures were predominantly found in the posterolateral regions of the discs. Adams (1982) demonstrated the propensity of slightly degenerated discs to prolapse posteriorly when hyperflexed and showed that fatigue failure might occur in lumbar discs as the outer posterior annulus is overstretched in the vertical direction while severely loaded in flexion. In an analytical study, interlaminar shear stresses, which can produce delaminations, have been found to be highest in the posterolateral regions of the disc (Goel 1995). These prior data indicate: 1) the posterior disc and posterior longitudinal ligament are at risk of degenerative changes, and that 2) the mechanism of degeneration can involve flexion fatigue.
To date, however, no treatments capable of reducing mechanical degradation to collagenous tissues currently exist. In fact, no other collagenous tissue fatigue inhibitors have been proposed. A need therefore exists for a method for improving the resistance of collagenous tissues in the human body to fatigue and for reducing the mechanical degradation of human collagenous tissues, in particular, the posterior annulus region of the intervertebral disc.